In water treatment using a separation membrane, in order to remove deposits on a membrane surface and suppress a rise in a transmembrane pressure difference, backwashing using membrane-treated water is performed. However, in order to degrade and remove deposits, which cannot be removed simply by backwashing, chemical washing using a sodium hypochlorite solution is performed periodically. When a separation subject is surface water, since raw water is clear, an interval of chemical washing is relatively long, but when a separation subject is sewage water, since much contaminant is contained in raw water, for example, chemical washing is performed at a frequency of around once per 2 hours.
This chemical washing is normally performed using washing water obtained by diluting a sodium hypochlorite solution, which has a high concentration, with membrane-treated water adjusted to be a predetermined hypochlorite concentration, as shown in Japanese Patent Application Laid Open (JP-A) No. 09-313902. As this dilution rate, a fixed value is used as far as a concentration of a normally used high concentration sodium hypochlorite solution is not changed. However, although a separation membrane is washed using diluted water, which has been diluted at a prescribed rate, the sufficient chemical washing effects cannot be obtained in some cases. In such cases, although chemical washing is performed, recovery of a transmembrane pressure difference of a separation membrane becomes insufficient, and stable operation cannot be performed.